During transport in a fluid, for example water or air, of a load suspended at a point of, for example, a crane, the load may be set in motion in an undesired way, for example during a start-up, a shutdown, a change of course, or an emergency shutdown of the crane. To avoid such unwanted movements, it is common to arrange rigid connections between the load and the crane. Another way of avoiding such unwanted movements is to work with slow acceleration and slow decelerations only. A trained and skilled crane operator may cancel an oscillating movement of the suspended load by achieving an oppositely directed movement with the aid of the crane. However, a mistake may lead to an amplification of the outward swing instead of its weakening thereof, which may cause a fatal result.
Transports of loads suspended from cranes, travelling cranes or the like, occur in many contexts; for example in connection with construction work, harbor work, or mining.
Another problem when working with suspended loads is when positioning them, especially when the load, which is suspended from, for example, a crane, is arranged at a large distance from the point of suspension. During positioning of such a load, it is normal, after the rough positioning, to finely adjust by controlling the crane with small movements to successively transfer the load to the correct position. This fine adjustment requires much experience and high professional skill of the crane operator to achieve the correct position in a relatively short time. If the crane operator lacks this experience and skill, the fine adjustment may be a very time-consuming work.
One specific application where a load is transported suspended with the aid of, for example, a travelling crane is in the shutdown of a nuclear power plant. A shutdown takes place, for example, when service needs to be carried out in the reactor vessel of the nuclear power plant. For this purpose, the reactor vessel is then emptied completely, or partially, of fuel assemblies and/or control rods which are normally arranged therein. The fuel assemblies and/or control rods are then transported between the reactor vessel and a so-called fuel pool where they are to be temporarily stored during the service. A gripping member is then usually arranged so as to be suspended from the travelling crane via a stiff telescopic arm. The gripping member is arranged with the aid of the travelling crane, and the telescopic arm, in position above a fuel assembly or a control rod, whereupon the gripping member is caused to grip the load in question, that is, the fuel assembly or the control rod. The telescopic arm is caused to be retracted whereby the load is then lifted up from the reactor vessel. The load is then transported, raised in this position, to a predetermined position in the fuel pool where, after the correct position has been found, it is lowered down into a so-called fuel rack on the bottom of the fuel pool and is detached from the gripping member.
In this application it is very important that the load is not set in motion in a fuel assembly, which, in the worst case, may result in fissionable material entering the reactor hall of the nuclear power plant.
This handling of fuel assemblies, and/or control rods is a separate operation which takes a relatively large proportion of the total shutdown time. Another time-wasting work operation is the positioning of the gripping member in a position within very close tolerances in order to be able to lift up a fuel assembly or a control rod from the reactor vessel. A corresponding positioning takes place when the load is to be positioned correctly to be able to be moved down into the fuel rack. This fine adjustment is usually done manually by transferring the travelling crane for positioning the gripping member which is arranged at a distance of approximately 10-20 meters from its suspension via the telescopic arm in the travelling crane.
The object of the present invention is to provide a method and a device which allow an accelerated and directionally stable transport with continuous correction of unwanted movements and simplified fine-positioning of suspended loads.